A technique used for evaluating formations surrounding an earth borehole is resistivity logging. Porous formations with high resistivity generally indicate the presence of hydrocarbons whilst porous formations with low resistivity are normally water saturated and contain no hydrocarbon.
Typically, tools used to measure formation resistivity during drilling use a propagated wave and two or more, but typically two, receiver antennas to measure phase delay and attenuation over a predefined interval. Such devices are described in detail in U.S. Pat. No. 3,551,797 and U.S. Pat. No. 4,968,940. All conventional devices for use during drilling use antennas on the outside of a drill collar to transmit or receive the signal.
Antennas on the outside of collars and mandrels are inherently unreliable, even with steel or fibreglass protective clamshell designs such as that disclosed in U.S. Pat. No. 4,536,714. The reason for this is that such protective covers rotate against the well bore wall and because they are attached to the collar by various fixing methods, they are prone to structural failure. Additionally with antennas mounted on the exterior of mandrels or drill collars, high pressure mechanical and electrical sealing systems are required to couple the antennas to the electronics mounted within the mandrel or collar in order to retain the integrity of the system. Such sealing systems are prone to failure and cause reliability problems.
Furthermore, antennas designs of this nature require radial drilling and reduction in diameter of the mandrel to accommodate the standoff which may be a ferrite or other non conductive, non permeable material. The removal and machining of this profile to accommodate the standoff weakens the body of the collar or mandrel considerably which means designs with small diameter collars are impractical. Also, conventional designs require the use of electrostatic shields which are cumbersome and space consuming within the antenna design.